Autosomal dominant polycystic kidney Disease (ARPKD) is characterized by formation and expansion of cystic dilations of the renal collecting duct leading to renal failure early in life. Over-expression and mislocalization of the epidermal growth factor receptor (EGFR) to the apical membrane of polarized cyst-lining renal epithelial cells is a common feature of several forms of PKD, including human ARPKD and mouse models of ARPKD. Studies have shown that EGFR function contributes directly to pathogenesis of several mouse models of ARPKD, including the orpk mouse. Preliminary data have focussed attention on the function of the abnormally-located apical EGFR. It is hypothesized that apical EGFR exhibits impaired down-regulation leading to prolonged activation of the mitogen-activated protein kinase (MAPK) pathway, which stimulates cell hyperproliferation, and to disruption of the phosphatidyl-3-kinase (PI3K) pathway, which is required for normal multicellular organization, thereby contributing to cyst formation/expansion. This hypothesis will be tested using the orpk mouse model. One set of experiments will compare down-regulation of apical versus basolateral EGFR by three mechanisms in confluent populations of primary renal collecting duct cells from normal versus orpk mice. A second set of experiments will examine the effect of stimulating apical versus basolateral EGFR on activation of MAPK and on proliferation of collecting duct cells and will determine if down-regulation-deficient EGFR can induce sustained MAPK activation and collecting duct cell hyperproliferation. A third set of experiments will examine the effect of activating apical versus basolateral EGFR on activation and localization of PI3K and Gab1 protein, which participates in the PI3K pathway regulating multicellular organization. The effect of manipulating MAPK and PI3K activities on multicellular organization into branching tubular (normal) versus cystic (abnormal) structures will be examined. These studies will determine if abnormal regulation/signalling by apical EGFR promotes altered collecting duct cell behaviors which contribute to cyst formation/expansion. They will provide the basis for development of treatment strategies targetting these pathways to interfere with early cystic development.